Temperature compensated optical measuring instruments

ABSTRACT

To reduce errors in a measuring instrument caused by temperature effects from changes in the flow rate of fluids passing through the instrument, the flow of heat is adjusted to balance the flow-rate-dependent temperature increases against the flow-rate-dependent temperature decreases that occur with the same change in flow rate in each flow path. Some of the techniques for adjusting the flow of heat are: (1) controlling the temperature of the fluid at the inlet of the instrument with one heat exchanger and the temperature of the body of the instrument with another heat exchanger; (2) controlling the temperature of the walls of the instrument with a heating coil; and (3) emitting heat within the flow path from a transducer. A typical measuring instrument in which such errors are reduced is a refractometer used to measure characteristics of a fluid flowing through a column.

This application is a continuation-in-part of U.S. patent applicationSer. No. 251,712 filed May 9, 1972, for Heat of Interaction Detector byMr. Robert W. Allington now U.S. Pat. No. 3,967,492.

This invention relates to measuring instruments and more particularlyrelates to apparatuses and methods for reducing errors from temperatureeffects caused by changes in the flow rates of fluids passing throughthe instruments.

In one class of measuring instrument, one or more passageways receive afluid or fluids, and measure some characteristic of the fluid generallyto determine the nature of substances carried by the fluid. The rate offlow of the fluid in the passageway or passageways are subject tovariations from the pumping sources and these variations may causeerrors in the measurements. It has been found that one cause of sucherrors results from temperature effects within the fluid.

One type of measuring instrument in which it has been known thattemperature effects cause errors due to flow rate changes is a heat ofinteraction detector. One type of heat of interaction detector, forexample, includes a passageway through which a fluid passes, aninteractive material within the passageway that interacts with aningredient of the fluid to absorb or release heat, and a temperaturemeasuring device in the vicinity of the interactive material to measurethe changes in temperature caused by the heat that is released orabsorbed. A second temperature measuring device is positioned to assumethe temperature of the fluid and serves as a reference, with thedifference between the two temperature measuring devices indicating thetemperature difference that is caused by the release or absorption ofheat. An arrangement is also included in some heat of interactiondetectors of this type to reduce flow-rate-dependent temperature errorsin the temperature measurement, which errors result from changes in thetemperatures that occur with changes in the flow rate of the fluidpassing through the heat of interaction detector.

In some prior art heat of interaction detectors of this type, thearrangement for reducing the flow-rate-dependent temperature errorsincludes wall portions of the passageway in the vicinity of the twotemperature measuring devices, which wall portions are of a materialhaving high thermal conductivity and which form a restricted diameterpassageway for the fluid. This arrangement is intended to remove heatfrom the fluid as it passes between the two temperature measuringdevices in such a quantity as to compensate for the flow-rate-dependenttemperature increases between the two temperature measuring devices. Anarrangement of this type is described in U.S. Pat. No. 3,535,918 to Munkand in a corresponding article by Munk and Raval entitled "FlowSensitivity of the Micro Adsorption Detector," published in the Journalof Chromatographic Science, Vol. 7, Jan. 1969, pp. 49-55.

The prior art arrangement for reducing the flow-rate-dependenttemperature errors has the disadvantages of not producing reliableresults and of being difficult to adapt to different carrier fluids,flow rates, and temperature conditions. The results of the prior artarrangement are unreliable in that the prior art arrangement reduces theflow-rate-dependent temperature errors by different amounts in anunpredictable manner under what appear to be the same conditions. Theprior art arrangement is difficult to adapt to different carrier fluids,flow rates and temperature conditions because it is not easilyadjustable, requiring in some cases the insertion of a different wallportion to provide a different thermal transfer characteristic for suchadjustment.

Other prior art heat of interaction detectors include apparatus by whichthe heat transfer characteristics of the wall portions are more easilyadjusted. For example, a calorimeter is disclosed in U.S. Pat. No.3,467,501 to Groszek having a heating coil around a temperature shieldin which the calorimeter is mounted to maintain the temperature shieldat a constant temperature when the calorimeter is operated at anelevated temperature. However, this type of prior art heat ofinteraction detector does not include apparatus specifically intended tocompensate for flow-rate-dependent temperature errors nor apparatus forautomatically controlling the heating element.

Accordingly, it is an object of the invention to provide a novelapparatus and method for reducing the flow-rate-dependent temperatureerrors in instruments.

It is a still further object of the invention to provide an apparatusand a method for balancing flow-rate-dependent temperature increasesagainst the flow-rate-dependent temperature decreases that occur withthe same change in flow rates in instruments that measurecharacteristics of fluids.

It is a still further object of the invention to provide an apparatusand a method for easily adjusting an instrument for measuring acharacteristic of a fluid to reduce the flow-rate-dependent errorscaused by temperature effects.

It is a still further object of the invention to provide an instrumentthat can be adjusted to reduce the flow-rate-dependent errors that arecaused by temperature effects without being disassembled and whileremaining in operation.

It is a still further object of the invention to provide an apparatusand a method for controlling the flow of heat within an instrument formeasuring a characteristic of a fluid.

It is a still further object of the invention to provide an instrumentfor measuring a characteristic of fluids that is insensitive toflucuations in ambient temperature and inlet stream temperature.

It is a still further object of the invention to provide a method andapparatus for reducing the flow-rate-dependent error caused bytemperature effects of an instrument for measuring a characteristic of afluid to a minimum by adjusting one or more of the heat flow paths inthe instrument.

It is a still further object of the invention to provide a method andapparatus for reducing the flow-rate-dependent error caused bytemperature effects of an instrument to a minimum by adjusting thedifference between the temperature of a fluid entering the inlet of theinstrument and the temperature of the body of the instrument.

It is a still further object of the invention to provide a method andapparatus for reducing the flow-rate-dependent temperature error of aninstrument for measuring a characteristic of a fluid to a minimum byadjusting the temperature of the walls of the instrument.

It is a still further object of the invention to provide a method andapparatus for reducing the flow-rate-dependent temperature error of aninstrument for measuring a characteristic of a fluid by adjusting theheat emitted by heating devices within the instrument for measuring acharacteristic of a fluid, which heating devices may be the temperaturemeasuring devices in heat of interaction detectors.

It is a still further object of the invention to provide a method andapparatus for reducing the flow-rate-dependent error of an instrumentfor measuring a characteristic of a fluid by adjusting the averagetemperature of the fluid in the instrument.

In copending patent application Ser. No. 251,712, a heat of interactiondetector is disclosed having an adjustable heat-flow control means.Before operating the heat of interaction detector to measure the heat ofinteraction of the active ingredient in the fluid and the interactivematerial, the flow-rate-dependent temperature error of the heat ofinteraction detector is detected and reduced to a minimum by adjustingthe heat-flow control means.

The heat-flow control means is adjusted while the flow rate is varieduntil the rate of change of the difference in the temperatures of thereference temperature measuring device and the heat of interactionmeasuring device is a minimum. At this point, the heat flow factors thatcause flow-rate-dependent temperature increases are balanced against thefactors that cause flow-rate-dependent temperature decreases with thesame magnitude of change in the flow rate of the fluid.

It has been discovered that a similar procedure reduces errors incertain other instruments which do not measure temperature directly and,indeed, in instruments which compare measurements of two different flowstreams, one being a reference flow stream and the other being a flowstream containing a substance a characteristic of which is to bemeasured. Even though the temperature is not directly measured in theseinstruments, errors are introduced in the measurement by fluctuations intemperature, which fluctuations are flow-rate-dependent. Such errors arenot removed by the comparison with measurements in the flow stream undersome circumstances.

In one embodiment, the heat-flow control means includes two heatexcahngers for each flow stream, one of which controls the temperatureof the inlet fluid to the instrument and the other of which controls thetemperature of the body of the instrument. The temperature of the fluidentering the instrument is adjusted with respect to the temperature ofthe body of the instrument until a minimum flow-rate-dependenttemperature error results.

When applied to a heat of interaction detector, there is a single flowstream and two heat exchangers which in cooperation with each othercontrol the temperature of the fluid entering the detector with respectto the temperature of the body of the detector. When applied to otherinstruments such as a refractometer, there are two flow streams, withheat exchangers to control the temperature of the fluid entering theinstrument with respect to the body of the instrument. In the case of arefractometer, the errors in refractive index of the fluids areflow-rate-dependent because of temperature changes caused by variationsof the flow rate in the two streams with respect to each other and theseerrors are reduced to a minimum by the temperature control system.

In another embodiment of the instrument, the temperatures of the fluidsat the inlets of the two flow streams of a refractometer areindividually controlled by heaters adjusted to the flow streams oractually in the flow streams. This embodiment enables each flow streamto be individually adjusted.

In still another embodiment, the heat-flow control means includes one ormore heating coils which impart an adjustable amount of heat to thewalls of the instrument to control their temperature. The temperature ofthe walls of the instrument is adjusted until the minimumflow-rate-dependent error results.

In still another embodiment which measures temperature directly, thetemperature measuring devices are thermistors and the heat-flow controlmeans is a circuit for controlling the heat emitted by the thermistors.With this arrangement, the temperature in the vicinity of one of thethermistors is adjusted with respect to the temperature in the vicinityof the other thermistor until a minimum flow-rate-dependent temperatureerror results.

With these structures and methods of operation, the instrument of thisinvention has several advantages over the prior art instruments formeasuring a characteristic of a fluid such as: (1) it providesconsistent and repreducible reductions in the flow-rate-dependenterrors; (2) it is less sensitive to fluctuations in the ambienttemperature and inlet stream temperature; and (3) it may be easilyadjusted without disassembling and without terminating its operation.

The above-noted and other features of the invention will be betterunderstood from the following detailed description when considered withreference to the accompanying drawings in which:

FIG. 1 is a simplified longitudinal sectional view of a portion of aheat of adsorption detector including an embodiment of the invention;

FIG. 2 is a schematic circuit diagram of another portion of a heat ofadsorption detector including another embodiment of the invention;

FIG. 3 is a schematic diagram of another embodiment of the invention;

FIG. 4 is a longitudinal sectional view of a model of a portion of aheat of adsorption detector illustrating one explanation of theoperation of the invention;

FIG. 5 is a graph illustrating the relationship between theflow-rate-dependent temperature error and the flow rate of the carrierfluid for a typical heat of adsorption detector;

FIG. 6 is a graph illustrating the relationship between the componentsof the flow-rate-dependent temperature error shown in FIG. 5 and theflow rate of the carrier fluid for a typical heat of adsorptiondetector;

FIG. 7 is a graph showing the experimentally-determined relationshipbetween the rate of change of the flow-rate-dependent temperature errorwith respect to changes in the rate of flow of the carrier and thedifference between the inlet temperature of the carrier fluid and thetemperature of the body of a heat of adsorption detector or arefractometer;

FIG. 8 is a graph showing the experimentally-determined relationshipbetween the rate of change of the flow-rate-dependent temperature withrespect to changes in the rate of flow of the carrier and the poweradded to thermistors in the embodiment of FIG. 2; and

FIG. 9 is a block diagram of a refractometer in accordance with theinvention.

STRUCTURE OF FIRST EMBODIMENT

In FIG. 1, there is shown, in a longitudianl sectional view, a basicportion of a heat of interaction detector 10 having an inlet section 12and an outlet section 14 positioned contiguous with each other andclamped together by any suitable clamping means (not shown), with acentral fluid passageway 16 passing through both sections 12 and 14. Aheat flow control means, which in one embodiment is a heating coil 15,is wound around the outer walls of the central portions of the inletsection 12 and the outlet section 14.

The heat of interaction detector 10 may be considered as a calorimeteror instrument for measuring the heat transferred as a result of thecontact between a fluid passing through the fluid passageway 16 and asolid or semisolid that is relatively stationary within the heat ofinteraction detector 10 and includes instruments for measuring any ofthe different heats of interaction, such as the heats of sorption,adsorption, absorption, preferential sorption, chemical reaction and thelike. Moreover, it may be used for other related purposes such as to:(1) determine the surface area of powders which release heat inproportion to their surface areas when they are contacted by certainfluids or (2) detect a specific chemical by incorporating an enzyme thatis specific to the chemical as part of the solid or semisolid, in whichcase the heat of the enzyme-catalyzed reaction provides a measure of theamount of the specific chemical in the fluid. However, although theprinciples of the invention have more general application, a heat ofadsorption detector will be specifically described hereinafter as thepreferred embodiment for the purposes of explanation.

The inlet section 12 includes an inlet support body 18, a cylindricalinlet tube 20, an inlet porous separating disc 22, an inert packingmaterial 24, a transverse, cylindrical, sleeved, reference-temperaturehole 26, a reference-temperature measuring device 28, which is athermistor in the preferred embodiment, and a cylindrical inlet supporthole 30.

To provide a path for the fluid to flow past the reference-temperaturemeasuring device 28, the cylindrical inlet support hole 30 passesthrough the inlet support body 18 from an inlet end to an inner end,where it communicates with the outlet section 14. The inlet porousseparating disc 22 is mounted near the center of the cylindrical inletsupport hole 30, having a tight fit against the internal walls thereof,with the cylindrical inlet tube 20 extending along the inlet supporthole 30 from one side of the inlet porous separating disc 22 through theinlet end of the support body 18 to receive the fluid and with the inertpacking material 24 being located in the support hole 30 between theother side of the inlet porous separating disc 22 and the inner end ofthe inlet support body 18 adjacent to the outlet section 14.

To provide a reference measurement of temperature, the transverse,cylindrical reference-temperature hole 26 extends through a centralportion of the inlet support body 18 in a direction perpendicular to thecylindrical inlet support hole 30, intersecting the cylindrical inletsupport hole 30 near the center of the inlet support body 18. Areference-temperature tube 32, having a smaller diameter than thecylindrical inlet support hole 30, fits tightly within thereference-temperature hole 26 and contacts the inert packing material24, which otherwise extends throughout the cylindrical inlet supporthole 30 between the inlet porous separating disc 22 and the central endof the inlet support body 18. The thermistor 28 is mounted within andcontacts the walls of the reference-temperature tube 32 near the centerof the cylindrical inlet support hole 30, with one conductive lead wire34 extending in one direction and the other conductive lead wire 36extending in the opposite direction along the transverse, cylindricalreference-temperature hole 26 out of diametrically opposite wallportions of the inlet support body 18.

The outlet section 14 includes an outlet support body 38, a centralporous separating disc 40, an interacting medium 42, a transverse,cylindrical, sleeved, temperature-measuring hole 44, atemperature-measuring device 46, which is a thermistor in the preferredembodiment, an outlet porous separating disc 48, a cylindrical outletsupport hole 50, and a cylindrical outlet tube 52.

To provide a path for the fluid to flow past the temperature-measuringdevice 46, the cylindrical outlet support hole 50 passes through theoutlet support body 38 from an inner end, where it communicates with thecylindrical inlet support hole 30 to an outlet end. At the inner end,the cylindrical outlet support hole 50 includes a counterbore into whichthe central porous separating disc 40 is fitted. The outlet porousseparating disc 48 is mounted near the center of the cylindrical outletsupporting hole 50, having a tight fit against the internal wallsthereof, with the cylindrical outlet tube 52 extending along the outletsupport hole 50 from one side of the outlet porous separating disc 48through the outlet end of the support body 38 to release the fluid andwith the interacting medium 42 being located in the support hole 50between the other side of the outlet porous disc 48 and the centralporous disc 40 adjacent to the inlet section 12.

To provide for the measurement of temperature, the outlet transverse,cylindrical temperature-measuring hole 44 extends through a centerportion of the outlet support body 38 in a direction perpendicular tothe cylindrical outlet support hole 50, intersecting the cylindricaloutlet support hole 50 near the center of the outlet support body 38. Atemperature-measuring tube 54, having a smaller diameter than thecylindrical outlet support hole 50, fits tightly within thetemperature-measuring hole 44 and contacts the interacting medium 42,which otherwise extends throughout the cylindrical outlet support hole50 between the central porous separating disc 40 and the outlet porousseparating disc 48. The thermistor 46 is mounted within and contacts thewalls of the temperature-measurement tube 54 near the center of thecylindrical outlet support hole 50, with one conductive lead wire 56extending in one direction and the other conductive lead wire 58extending in the opposite direction through the transverse, cylindricaltemperature-measuring hole 44 and out of diametrically opposite wallportions of the outlet support body 38.

The specific materials used for the inert packing material 24, theporous discs 22, 40, and 48 and the interactive material 42 are selectedfor their effect or lack of effect with the active ingredient in thefluid that is forced through the central passageway 16 in a manner knownin the art. The interactive material may be, for example, activatedsilica gel or aluminum oxide to detect the heat of adsorption of organiccompounds in the fluid for the purpose of identifying organic compoundsin an organic solution from the heat of adsorption given off by thecontact between the compounds and the interactive material. The inertpacking material may be smooth glass beads, because little heat ofadsorption is provided by adsorption of most organic compounds on smoothglass beads. Since every material has a definite heat of adsorption withevery other material, there are, of course, many alternative materialswhich may be used for the interactive material and the inert materialsin the embodiment that is a heat of adsorption detector and stillfurther materials in other embodiments using the principles of thisinvention.

OPERATION OF FIRST EMBODIMENT

One use of the heat of adsorption detector 10 is to detect differentseparated zones of different organic compounts within a fractionatedliquid or gas mixture from the time of occurrence of their heats ofadsorption and desorption as they pass through the central fluidpassageway 16 and to actuate a fraction collector to collect each orcertain of the zones into different containers as described in U.S. Pat.No. 3,151,639 to Robert W. Allington. Another use of the heat ofadsorption detector 10 is to determine the quantity of organic compoundsfrom the magnitude of the detected heats of adsorption and desorption.For example, it may be used to identify, locate and determine thequantity of the different eluates in a carrier solvent after elutionchromatography to aid in the collecting of the eluates.

Before using the heat of adsorption detector 10 for either of thesepurposes, an interacting material 42 is selected, which interactingmaterial must release substantial amounts of heats of adsorption whencontacted by the eluates that are to be detected. The interactingmaterial 42 is assembled with the rest of the parts of the detector asshown in FIG. 1 and described above.

In the operation of the heat of adsorption detector 10 the heat flowcontrol means 15 is first adjusted to reduce the sensitivity of the heatof adsorption detector 10 to changes in the rate of flow of the carrierliquid and then the carrier liquid and eluates are caused to flowthrough the fluid passageway 16 to generate signals as each eluate isadsorbed and desorbed on and from the interacting material, whichsignals identify, locate, and determine the quantity of the differenteluates or types of eluates in carrier so as to permit their collectionby fraction collectors or determinations of their quantities by theamplitudes of the waveforms generated by the heat of adsorption detector10.

To adjust the heat flow control means 15, pure carrier fluid withouteluates is passed through the central fluid passageway 16. As thecarrier flows through the heat of adsorption detector 10, the electricalcurrent conducted through the heater coil 15, which is the heat flowcontrol means in the embodiment of FIG. 1, is adjusted to change theamount of heat flowing through the walls of the heat of interactiondetector 10 and thus change the temperature of the carrier fluid betweenthe thermistors 28 and 46. While the temperature of the carrier fluid isbeing adjusted, the rate of flow of the fluid is varied cyclically andthe electrical potentials from the thermistors 28 and 46 are compared ina conventional thermistor bridge circuit (not shown in FIG. 1) andobserved at each adjustment of the heater current to determine themagnitude of the variation of the output potential from the thermistorsas the rate of flow of the carrier liquid changes. A setting of the heatflow control means 15 is found by this procedure for which thedifference in potential between the outputs of the two thermistors 28and 46 is relatively constant as the rate of flow of the fluid changesand this setting maintained.

After the heat flow control means 15 has been properly adjusted toreduce the sensitivity of the heat of adsorption detector 10 to changesin the flow rate of fluid passing through the central fluid passageway16, the carrier and eluates in the chromatograph column are passedthrough the central fluid passageway 16 and the difference between thetemperatures of the two thermistors 28 and 46 is measured to locate thefluid stream or to identify the eluates.

As the carrier fluid passes through the central fluid passageway 16, itcarries eluates that have been separated into zones one by one throughthe inlet section 12 and the outlet section 14.

To generate a reference potential, the fluid in the inlet section passesthrough the cylindrical inlet tube 20, the cylindrical inlet porousseparating disc 22, and the inert packing material 24 of the inletsection 12. As the eluates pass through the inert packing material 24,they do not release any substantial heat of adsorption so that the inertpacking material 24 assumes substantially the temperature of the carrierfluid. Because the reference-temperature tube 32 is relatively small andhas a relatively high heat conductivity, it assumes the temperature ofthe inert packing material which is substantially the same temperatureas the carrier fluid so that the reference thermistor 28 provides arelatively constant amplitude output signal serving as a reference thatindicates the temperature of the carrier fluid and eluates.

To generate an electrical waveform indicating the beginning of an eluatein the liquid carrier, the liquid carrier in the outlet section 14passes through the central porous separating disc 40, the interactingmedium 42, the outlet porous separating disc 48 and the cylindricaloutlet tube 52.

As each eluate flows through the interacting medium, four processes takeplace, each generating a different portion of a two-part (positive andnegative) waveform that locates the beginning and identifies the natureof the eluate in the carrier fluid.

Firstly, as the eluate flows into the interacting medium 42, theinteracting medium adsorbs it, releasing the heat of adsorption to raisethe temperature of the interacting medium, the temperature-measuringtube 54 and the thermistor 46, which provides an increasing amplitudeoutput signal difference between the thermistors 28 and 46 correspondingto the heat of adsorpiton of the eluate on the interacting medium 42.This increasing amplitude output signal indicates the beginning locationof the eluate and is the leading edge of the positive part of thewaveform that indicates the eluate.

Secondly, after the eluate has reached equilibrium on the interactingmedium 42, the interacting medium stops adsorbing the eluate, resultingin no further release of heat and in a temperature of the interactingmedium 42, the temperature measuring tube 54 and the thermistor 46 thatis reduced by the flow of new liquid carrier so as to provide a fallingpotential difference between thermistors 28 and 46. This fallingpotential is the trailing edge of positive part waveform which indicatesthe eluate.

Thirdly, when the eluate has passed the interacting medium, theinteracting medium 42 desorbs the eluate into the relatively pure liquidcarrier that separates different eluates so as to remove heat from theinteracting medium, reducing the temperature of the interacting medium,the temperature measuring tube 54 and the thermistor 46 and resulting ina potential difference between the thermistors 28 and 46 that increasesin the opposite direction (thermistor 46 being cooler). This potentialdifference indicates that the eluate has passed the outlet section 14and is the leading edge of the negative part of the waveform thatindicates the eluate.

Fourthly, after the eluate has been desorbed, the temperature of theinteracting medium, the temperature measuring tube 54 and the thermistor46 is increased to the temperature of the carrier fluid by the carrierfluid resulting in a decreasing potential difference between thethermistors 28 and 46, which is the trailing edge of the negative partof the waveform that indicates the eluate.

The next eluate zone to flow into the heat of adsorption detector 10causes these four processes to repeat, but the time duration and theamplitude of the signals generated by the thermistor 46 may be differentbecause the heat of adsorption of the new eluate or the width andconcentration of the new zone may be different from the previous eluateand zone. The new waveform has a leading edge indicating the location ofthe beginning of the next eluate in the chromatographic column and othercharacteristics indicating the quantity of the eluate.

Accordingly, a plurality of waveforms are generated by the heat ofabsorption detector, each indicating the location of a different eluateand the quantity of the eluate which waveforms may be used to control afraction collector and to help determine the quantity of the eluates.

As the carrier and eluates flow through the heat of adsorption detector10, there are some temperature changes between the reference thermistor28 and the measuring thermistor 46 that are not due to the adsorption ordesorption of the eluates but arise from other unintentional causes,some of the other unintentional causes being related to the rate of flowof the carrier through the heat of adsorption detector. For example heatis conducted through the walls of the heat of adsorption detector at arate that is partly dependent on the flow rate of the liquid carryingthe heat. Also, heat generated by the thermistor 28 and by viscousfriction between the carrier and the material in the central fluidpassageway 16 is carried from upstream to the downstream thermistor 46at a rate dependent upon the rate of flow of the carrier.

Some of these unintentional, flow-rate-related causes of temperaturedifferences between the thermistors 28 and 46 increase the temperaturedifference in a first direction and some increase the temperaturedifference in a second direction, opposite to the first direction as theflow rate changes in any one direction.

The adjustment of the heat flow control menas 15 that was performed atthe start of the operation of the heat of adsorption detector 10balances the unintentional flow-rate related causes that increase in thefirst direction against those that increase in the second direction asthe flow rate changes so that the temperature remains relatively stableas the flow rate varies.

STRUCTURE OF SECOND EMBODIMENT

In FIG. 2, there is shown, in a schematic circuit diagram, a bridgecircuit 60 useful in indicating the difference between the potential ofthe thermistors 28 and 46 of the heat of adsorption detector 10 (FIG.1), with precision. The circuit of FIG. 2 also includes anotherembodiment of heat flow control means 62 which may be used in place ofor together with the heater 15 shown in the embodiment of FIG. 1.

To compare the resistances of the thermistors 28 and 46, the bridgecircuit 60 includes: (1) an output circuit 64; (2) two inactive arms,each including a different one of the two resistors 66 and 68; and (3)two active arms, each including a different one of the two thermistors28 and 46, with each of the resistors 66 and 68 and the thermistors 28and 46 having a different first lead wire and a different second leadwire.

To provide an output potential indicating the difference in thetemperatures of the two thermistors 28 and 46, the output circuit 64 ofthe bridge circuit 60 includes a differential amplifier 70, a feedbackresistor 72 connected between the output terminal and the invertinginput terminal of the differential amplifier 70, an input resistor 74connected at one end to the inverting input terminal of the differentialamplifier 70 and at its other end to the first lead wire of the resistor66 and to the first lead wire of the thermistor 28, and a meter 76connected between the output terminal of the differential amplifier 70and ground, the positive input terminal of the differential amplifierbeing connected to the first lead wire of the resistor 68 and to thefirst lead wire thermistor 46.

The differential amplifier 70 is a high gain operational amplifier thatcompares the potentials across the thermistors 28 and 46 and provides anoutput potential that indicates the relative difference in temperaturebetween the thermistors, with the resistors 72 and 74 stabilizing thegain of the amplifier 70.

To supply power to the bridge circuit 60 and to balance it, a variableresistor 78 has one end connected to the second lead wire of theresistor 66 and the other end connected to the second lead wire of theresistor 68, with a source of negative potential 80 being electricallyconnected to the adjustable tap of the potentiometer. To complete thebridge circuit 60, the second lead wire of the thermistor 28 isconnected to one end of a secondary winding 82 of a transformer 84 andthe second lead wire of the thermistor 46 is connected to one end of asecondary winding 86 of a transformer 88, with the other ends of thesecondary windings 82 and 86 of the transformers 84 and 88 beingconnected together and to ground. The tap on the variable resistor 78 isadjusted to balance the bridge.

The transformers 84 and 88 are part of the heat flow control means 62,which will be described hereinafter and their secondary windings 82 and86 have low resistance so that they do not materially affect theoperation of the bridge circuit 60. While a meter 76 is connected to theoutput of the differential amplifier 70 in the embodiment of FIG. 2,other types of indicating and recording instruments may be includedinstead and the output may also activate a fraction collector to collectzones of the chromatographic column into different containers.

To control the flow of heat within the heat of adsorption detector 10(FIG. 1), the heat control means 62 (FIG. 2) includes an alternatingcurrent generator 90, a resistor 92, a potentiometer 94, thetransformers 84 and 88, and two capacitors 96 and 98. The transformer 84includes a primary winding 100 having a first and electrically connectedto one end of the potentiometer 94 and the transformer includes aprimary winding 102 having a first end connected to the other end of thepotentiometer 94, with the second ends of the primary windings 100 and102 being electrically connected together and to ground.

To permit the flow of power to be directed more to one transformer thanthe other of the transformers 84 and 88, the adjustable tap of thepotentiometer 94 is electrically connected to the generator ofalternating current 90 through resistor 92. To provide an alternatingcurrent return from the bridge circuit 60, the capacitor 96 has oneplate connected to the first lead wires of the resistor 66 and thethermistor 28 and its other plate grounded and the capacitor 98 has oneplate connected to the first lead wires of the resistor 68 and thethermistor 46 and its other plate grounded.

OPERATION OF SECOND EMBODIMENT

In the operation of the bridge circuit of FIG. 2, the variable resistor78 is adjusted with a pure carrier liquid flowing through the centralfluid passageway 16 (FIG. 1) of the heat of adsorption detector 10 untilthe meter 76 is nulled. When the chromatographic carrier solvent flowsthrough the central passageway 16, the meter remains nulled until aneluate reaches the interacting medium 42.

When the eluate reaches the interacting medium 42, it is adsorbedthereon and increases the temperature of the thermistor 46, therebyreducing its resistance. As the resistance of the thermistor 46 falls, apositive going potential is applied to the positive input terminal ofthe differential amplifier 70 (FIG. 2) by the bridge circuit 60. Thispositive going potential is amplified by the differential amplifier 70and applied to the meter 76, which provides a positive deflection.

After equilibrium between the fluid and the interacting medium 42 isreached so that no more eluate is adsorbed, no further heat is releasedand the temperature of the thermistor 46 falls approximately to solventtemperature so that the output potential from the differential amplifierfalls to provide a negative-going potential, returning the meter to itszero deflection.

When the eluate has passed the interacting medium 42 and relatively puresolvent is flowing through the interacting medium 42, the interactingmedium desorbs the eluate, removing heat from the temperature measuringtube and the thermistor 46. This causes a further negative-goingpotential to be applied to the positive terminal of the differentialamplifier 70, resulting in the negative deflection of the meter 76.After the eluate has been desorbed, the temperature of the interactingmedium 42 returns to the temperature of the solvent, returning the meterto its zero deflection condition.

To control the flow of heat within the heat of adsorption detector 10,alternating current flows from the alternating current generator 90 andthe resistor 92 of the heat flow control means 62 through two paths,which are: (1) through a portion of the resistance in the potentiometer94 and the primary winding 102 of the transformer 88 in series in theorder named with a return through ground in the primary circuit of thetransformer 88; or (2) through a portion of the resistance in thepotentiometer 94 and the primary winding 100 of the transformer 84 inseries in the order named, with a return through ground in the primarycircuit of the transformer 84. The current through the primary winding102 of the transformer 88 induces an alternating current potential inthe secondary winding 86 of the transformer 88 which causes current toflow through and heat the thermistor 46 and current through the primarywinding 100 of the transformer 84 induces an alternating currentpotential in the secondary winding 82 of the transformer 82 which causescurrent to flow through and heat the thermistor 28 without affecting thed.c.-responding output circuit 64. There is a return to ground for thesecurrents through the capacitors 96 and 98.

To adjust the flow of heat within the heat of adsorption detector 10,the amount of the heat generated by each of the thermistors 28 and 46 isadjusted by adjusting the potentiometer 94 to increase the resistance inone of the paths and decrease it in the other path of the alternatingcurrent power from the alternating current generator 90, thus adjustingthe difference between the currents through the two thermistors 28 and46. The direction and the amount of the flow of heat is adjusted byshifting the relative amounts of heat generated by these currents andreleased by the two thermistors 28 and 46.

To control the heat flow so as to reduce the sensitivity of the heat ofabsorption detector to the rate of flow of the carrier, the heat flowcontrol means 62 is adjusted with pure carrier flowing through the heatof adsorption detector 10 in much the same manner as the heat flowcontrol means 15 shown in FIG. 1 is adjusted. To make this adjustment,the rate of flow of the pure carrier is varied at each of severalselected adjustments of the potentiometer 94, until an adjustment of thepotentiometer 94 results in a minimum amount of deflection of the meter76 as the rate of flow of the carrier is varied. This embodiment canalso be physically realized by placing separate electrically adjustableheating elements adjacent to the thermistors instead of using thethermistors themselves as heating elements.

STRUCTURE OF THIRD EMBODIMENT

In FIG. 3 there is shown another embodiment of the invention having aheat flow control means 104 connected to the heat of adsorption detector10. The heat flow control means 104 may be used by itself or with eitheror both of the heat flow control means 15 and 62 shown in FIGS. 1 and 2.

The heat flow control means 104 includes a first enclosure 106, a firstheat exchanger 108, a first temperature control apparatus 110B, a secondenclosure 112, a second heat exchanger 114 and a second temperaturecontrol apparatus 110A.

The second enclosure 112 preferably includes in its interior a materialthat has a high degree of thermal conductivity such as water or solidmetal so as to provide a uniform temperature throughout. Both enclosures106 and 112 have insulative casings and are fabricated in any mannerknown in the art for easy assembly consistent with the general purposeof controlling the temperature of apparatus within their interiors.

The temperature control apparatuses 110A and 110B each include acorresponding one of two inverting amplifiers 116A and 116B, twothermistors 118A and 118B, two resistance heaters 120A and 120B, twodiodes 122A and 122B and two potentiometers 124A and 124B. In each ofthese apparatuses, a source of positive potential 126 is electricallyconnected to two corresponding parallel circuits through a correspondingone of the thermistors 118, which two parallel circuits includecorresponding ones of: (1) the potentiometers 124 and a source ofnegative potential 128 in series in the order named; and (2) theamplifiers 116, the anodes of the diodes 122, the cathodes of the diodes122, the resistance heaters 120 and ground in series in the order named.

The heat of adsorption detector 10, the second heat exchanger 114, thethermistor 118A and the resistance heater 120A are within the secondenclosure 112 and maintained at the same temperature thereby; the firstheat exchanger 108, the thermistor 118B and the resistance heater 120Bare in the first enclosure 106 and maintained at the same temperaturethereby.

The purpose of the first and second heat exchangers 108 and 114 is toimpart a predetermined temperature to the fluid flowing from thechromatograph column before it enters the heat of adsorption detector 10with respect to the body of the heat of adsorption detector. In thepreferred embodiment, the first heat exchanger 108 is relatively longcompared to the second heat exchanger 114 and the first enclosure 106 iswarmer than the second enclosure 112 with the result that the fluidassumes the temperature of the first enclosure 106 before entering thesecond heat exchanger 114, which reduces the temperature some before itenters the heat of adsorption detector but not to the temperature withinthe second enclosure 112 so that it has a different temperature thanthat of the body of the heat of adsorption detector 10. However, otherarrangements can be used to create a predetermined temperaturedifference between the fluid and the body of the heat of adsorptiondetector 10, and, indeed, the second heat exchanger 114 could even bedispensed with under some circumstances.

To conduct the carrier fluid through the first and second heatexchangers 108 and 114 and the heat of adsorption detector 10, an inletconduit 130 communicates at one end with the outlet of the chromatographcolumn and at its other end with the heat exchanger 108 and a connectingconduit 132 communicates at one end with the first heat exchanger 108and at its other end with the second heat exchanger 114, the second heatexchanger 114 communicating with the heat of adsorption detector 10through the inlet tube 20.

OPERATION OF THIRD EMBODIMENT

In the operation of the heat flow control means 104, the heat ofadsorption detector 10 operates normally once the heat flow controlmeans 104 has been adjusted for minimum change in the temperatureresponse of the thermistors 28 and 46 (FIGS. 1 and 2) when the rate offlow of the carrier fluid is changed.

While the heat of adsorption detector 10 is operating, the temperaturesof the enclosures 112 and 106 are maintained constant at the same or atdifferent temperatures by the temperature control circuits 110A and 110Band the temperatures of the heat of adsorption detector 10 and thesecond heat exchanger 114 are maintained the same by the heat transfermedium within the enclosure 112. With this arrangement, the heat-flowcontrol means 104 can be adjusted to render the output waveform from theheat of adsorption detector substantially independent of changes of therate of flow of the carrier fluid within the normal range of changescaused by the pump for the carrier fluid. This is basically done byadjusting the difference between the temperature of the carrier fluid asit enters the heat of adsorption detector 10 and the temperature of thebody of the heat of adsorption detector.

The temperature control circuits 110A and 110B operate in the samemanner to maintain the temperature of their respective enclosuresconstant and the operation of only one 110A of the temperature controlcircuits 110A and 110B will be described herein.

If the temperature within the enclosure 112 changes from the settemperature, the thermistor 118A senses the change and applies a signalto the amplifier 116A which adjusts the current flow through theresistance heater 120A to compensate for the change. If the temperaturewithin the enclosure 112 is reduced below the set temperature, theresistance of the thermistor 118A is increased, resulting in a negativechange in the potential at the input of the inverting amplifier 116A.The increase in the negative potential at the input of the invertingamplifier 116A results in an increased positive potential being appliedto the resistance heater 120A through the diode 122A, thus supplyingmore heat to the enclosure 112. Similarly, an increase in thetemperature above the set temperature within the enclosure 112 causes areduction in the resistance of the thermistor 118A and an eventualdecrease in the potential applied to the resistance heater 120A toreduce the temperature within the enclosure 112.

To adjust the heat flow control means 104, the potentiometer 124B isadjusted to different settings while the rate of flow of pure carrierfluid through the heat exchangers 108 and 114 and the heat of adsorptiondetector 10 is varied until a setting is found for which there is aminimum change in the difference between the temperature of thethermistors 28 and 46 (FIGS. 1 and 2) corresponding to the change in therate of flow of the carrier. With this adjustment, the detection andidentification of eluates by the heat of adsorption detector isunhindered by changes in the temperature of the thermistors 28 and 46caused by changes in the rate of flow of fluid from the chromatographcolumn.

THEORY OF OPERATION

It is not completely understood why the means for controlling the flowof heat in a heat of adsorption detector reduces the sensitivity of theheat of adsorption detector to variations in the flow rate of thecarrier through the heat of adsorption detector, but one explanation canbe obtained from an analysis of the model shown in FIG. 4 of the heat ofadsorption detector 10.

The model shown in FIG. 4 assumes that:

(1) thermistors 28 and 46 are located in a single cylindrical fluidpassageway 16 within a single support body;

(2) the fluid passageway 16 is packed with a medium which has thermalconductivity properties and viscous friction properties which areconstant throughout the length of the passageway, with fluid flow beingfrom left to right in the passageway;

(3) the temperatures of the items listed in the first column of Table 1are represented by the symbols in the second column;

(4) in Table 2, the differences between the temperatures listed in thefirst column are represented by the symbols in the second column;

(5) the heats listed in the first column of Table 3 are represented bythe symbols listed in the second column; and

(6) the symbol f represents the rate of flow of the carrier fluid and xis a point in the carrier fluid between the thermistors 28 and 46.

With the above assumptions, equations representing partial changes canbe written from the model of FIG. 4 in the form T_(n), m = T_(m) -T_(o)due to the heat flow n. For example, Tl,a is T_(a) due to, and only to,heat flow H₁. Equations (1) - (4) are four sets of such equations.

By summing equations (1) - (4), equation (5) is written, describing thetemperature difference T between the thermistors 28 and 46.

By differentiating T with respect to f, an expression shown in equation(6) is obtained that describes the rate of change of the temperaturedifference T between the thermistors 28 and 46 caused by variations inthe rate of flow f of the carrier fluid.

                  TABLE 1    ______________________________________                          symbol for    items                 temperature    ______________________________________    fluid entering the    passageway            T.sub.i    support body          T.sub.o    thermistor 28 and    its vicinity          T.sub.a    thermistor 46 and    its vicinity          T.sub.b    carrier fluid at point between    thermistors 28 and 46 T.sub.x    average temperature of carrier    fluid between thermistors 28    and 46                T.sub.av    ______________________________________

                  TABLE 2    ______________________________________    difference in    temperatures:         symbol    ______________________________________    T.sub.a - T.sub.o     T.sub.a--    T.sub.x - T.sub.o     T.sub.x--    T.sub.b - T.sub.o     T.sub.b--    T.sub.av - T.sub.o    T.sub.av---    T.sub.b - T.sub.a---- T--    T.sub.i - T.sub.o     T.sub.i--    ______________________________________

                  TABLE 3    ______________________________________    heat leaving the vicinity of    the thermistor 28 due to    electric heating of the    thermistor 28              H.sub.1    heat leaving the fluid carrier    through the support body as the    fluid carrier flows between    thermistor 28 and thermistor 46                               H.sub.2    heat added to the fluid carrier    by viscous friction while flow-    ing between the thermistor 28    and the thermistor 46      H.sub.3    heat leaving the vicinity of    the thermistor 46 due to elec-    tric heating of the thermistor 46                               H.sub.4     ##STR1##     ##STR2##     ##STR3##     ##STR4##     ##STR5##     ##STR6##    where k.sub.1 is a constant relating flow to temp-    erature rise, primarily related to the heat    capacity of the carrier fluid; k.sub.2 is analagous    to k.sub.1 ; and k.sub.3 is a coefficient related to    viscous friction of the fluid in the station-    ary medium    ______________________________________

Equation (6) indicates the reasons that enable the heat flow controlmeans 15, 62, 104 to decrease the flow-rate-related error in thetemperature readings of the heat of absorption detector.

In equation (6), one of the flow-dependent terms is positive and one isnegative. Moreover, one of the terms includes the factor T_(av) - fdT_(av) /df and the sign of this factor is changeable. The flow ratedependent terms of the equation are adjusted by the heat control means15, 62, and 104 to vary by approximately the same amounts in oppositedirections as the rate of flow of the carrier fluid changes so as toreduce the flow-rate-related temperature error in the readings.

The term f dT_(av) /df in the above factor is generally insignificantfor two reasons, which are: (1) the average temperature difference of avolume of fluid inherently changes more slowly than the temperature at apoint in the volume so that dT_(av) /df will be small for larger changesin the temperature at specific points in the heat of adsorptiondetector; and (2) when properly adjusted, the heat of adsorptiondetector 10 is operated at points of minimum dT/df as shown in FIGS. 7and 8 and described hereinafter and these are inherently close to pointsof minimum dT_(av) /df.

Moreover, experience shows that the embodiments described in this casebehave as if f dT_(av) /df were smaller than T_(av) or has the oppositesign from T_(av). Logically one would expect the sign of f dT_(av) /dfto be opposite to that of T_(av), so that - f dT_(av) /df will have thesame sign as +T_(av), thereby making these two terms additive andmutually effective. This is because T_(av) due to H₁ decreases as flowincreases thus tending to make f dT_(av) /df negative and therefore - fdT_(av) /df positive.

Each of the three heat flow control means 15, 62, and 104 is an exampleof a different manner of controlling the flow of heat in the heat ofadsorption detector 10 to cause the carrier flow-rate-dependent terms ofthe equation (6) to vary in opposite directions by approximately thesame amounts as the flow rate of the carrier changes, or stated anotherway, is an example of a different manner of balancing theflow-rate-dependent terms of the equation.

Firstly, in the embodiment of FIG. 1, the flow-rate-dependent terms ofequation (6) are balanced by changing the term containing T_(av), whichis accomplished by changing the amount of heat imparted to the walls bythe heat control means 15 and thereby changing the temperature T_(o) ofthe walls of the heat of adsorption detector 10, T_(av) being thedifference between T_(o) and the fluid temperature T_(av).

Secondly, in the embodiment of FIG. 2, the flow-rate-dependent terms ofequation (6) are balanced by changing the two flow related terms inopposite directions, which is accomplished by adjusting thepotentiometer 94 of the heat control means 62 to: (1) change in a firstdirection the amount of heat H₁ released by the upstream thermistor 28and thereby change T_(av) and the term of equation (6) containing T_(av)in a first direction; and (2) change the amount of heat H₄ released bythe downstream thermistor 46 in a second direction and thereby changethe term of equation (6) containing H₄ in the second direction.

Thirdly, in the embodiment of FIG. 3, the flow-rate-dependent terms ofequation (6) are balanced by changing T_(av), which is accomplished byadjusting the heat control means 104 to change the inlet termperatureT_(i) of the fluid entering the passageway 16, a change that can causeT_(av) to become either positive or negative by changing T_(i) to avalue above or below T_(o).

FIGS. 5-8 provide another illustration of the manner in which the heatflow control means 15, 62, and 104 reduce the flow-rate-dependenttemperature error of the heat of adsorption detector 10.

In FIG. 5, there is shown a graph 130 having ordinates of spurious T oferror in the measured temperature difference between the referencethermistor 28 and the measuring thermistor 46 and abscissae of flow rateof the carrier fluid in the central fluid passageway 16. In the graph130, a curve 132 of the relationship between the spurious T and the flowrate of the carrier fluid has portion 134 of rapidly increasing T withincreasing flow rate which portion 134 occurs at relatively low flowrates, a portion 136 of maximum T which starts at the end of the portion134 of rapidly increasing T, a portion 138 of rapidly decreasing T withincreasing flow rate, at flow rates higher than those at which themaximum T occur.

In FIG. 6, there is shown another graph 142 having ordinates of spuriousT and abscissae of flow rate with three curves, showing the relationshipbetween three of the causes of the variations of the spurious T of theheat of adsorption detector 10 with respect to the flow rate illustratedby curve 132 in FIG. 5 which three curves are: (1) curve 144 showing therelationship between spurious T caused by loss of heat through the wallsof the heat of adsorption detector 10, which is considered a negativespurious T since it causes the measuring thermistor 46 to be cooler; (2)curve 146 showing the relationship between the spurious T caused by therelease of heat from the second thermistor, which is a positive spuriousT since it causes the measuring thermistor 46 to be warmer than thereference thermistor 28; and (3) curve 148 showing the relationshipbetween the spurious T caused by viscous friction, which is a positive Tsince it causes the measuring thermistor 46 to be warmer than thereference thermistor 28.

The portions 134, 136, 138 and 140 of the curve 132 in FIG. 1 can beunderstood from the curves 144, 146 and 148 of the graph 142.

Firstly, the rapidly increasing T portion with increasing f 134 (FIG. 5)of the curve 132 is caused by a more rapid decrease in the heat lost tothe walls as the flow rate increases as shown by curve 144 (FIG. 6) thanthe decrease in the temperature rise due to heat released by the secondthermistor as shown by the curve 146 (FIG. 6). The rapid increase in theheat lost to the walls at lower flow rates as the flow rate decreaseshas two causes, which are: (1) the carrier fluid is between thereference thermistor 28 and the measuring thermistor 46 for a longerperiod of time permitting a greater transfer of heat from the carrierfluid through the walls; and (2) the slow rate of flow of the carrierfluid increases the temperature rise of the carrier fluid due to theincreased time available for the transfer of heat from the referencethermistor 28 to each unit volume of the carrier fluid, with thisincrease in temperature causing more heat to be lost.

The portion 136 of the curve 132 occurs when the negative spurious Tcaused by heat loss through the walls and the positive spurious T causedby the heat released by the measuring thermistor 46 decrease at the samerate as shown by curves 144 and 146. This portion is of special interestbecause the total T is relatively constant with varying flow rates whichcause flow-rate-dependent errors to be a minimum.

The portion 138 of rapidly decreasing T with increasing flow rates ofthe curve 132 occurs at flow rates for which the positive spurious Tfrom the heating of the measuring thermistor 46 decreases at a fasterrate than the negative spurious T from the heat lost to the walls. Thisoccurs because the flow rate is sufficiently large to cool boththermistors without causing a large increase in the average temperatureof the carrier fluid between the reference thermistor 28 and themeasuring thermistor 46 and because the carrier fluid moves between thetwo thermistors faster so that it does not lose as much heat through thewalls of the heat of adsorption detector.

The portion 140 (FIG. 5) of curve 132 for slowly decreasing spurious Twith increasing flow rate occurs because the slowly increasing spuriousT caused by viscous friction shown by curve 148 (FIG. 6) becomessufficiently large to compensate partly for the decrease in thedifference between the spurious T from heat loss through the walls andthe spurious T from heat released by the temperature measuringthermistor 46.

The heat-flow control means 15, 62, and 104 have the effect of moving orbroadening the portion 136 of the curve 132 to cover a wider range offlow rates. Without the heat-flow control means the heat of adsorptiondetctor 10 is unlikely to operate in the portion 136 of curve 132.

In FIGS. 7 and 8 there are shown graphs 150 and 152 respectively ofexperimental results from testing the embodiments of the heat ofadsorption detectors shown in FIGS. 3 and 2, respectively, each havingordinates of the rate change, dT/df, of spurious T magnitude and phasewith respect to a flow rate f, with the graph 150 having abscissae T_(i)of the difference between the temperature T_(i) of the carrier fluidentering the central fluid passageway and the temperature T_(o) of thewalls of the heat of absorption detector 10 and with the graph 152having abscissae of units of power added to the reference thermistor 28.In making the measurements for these graphs, an average flow rate of 50ml/hr with a 5 percent variation in the flow rate at 0.5 cpm was used toapproximate df for the graph 150 and an average flow rate of 50 ml/hrwith a 6 percent variation in the flow rate at 0.5 cpm was used toapproximate df for the graph 152.

These graphs provide a further illustration of the operation of theheat-flow control means when considered with equation (6) and the graphs130 and 142 of FIGS. 5 and 6.

The graph 150 of FIG. 7 includes a curve 154 indicating the relationshipof the magnitude of variation in the spurious dT/df and a curve 156indicating the relationship of the phase of the spurious dT/df withrespect to variations in the input temperature T_(i) of the carrierfluid.

The curve 154 includes a portion 158 of declining spurious dT/df and aportion 160 of increasing spurious dT/df, with the portion 158 ofdeclining dT/df declining from a negative input temperature to aslightly positive temperature indicated at 162 and with the portion 160of increasing dT/df increasing from the temperature at 162 to a positivetemperature, the slightly positive temperature at 162 being thetemperature of the input carrier fluid that provides the least variationin T with changes in flow rate.

The curve 156 includes a portion 164 of negative phase for dT/df and aportion 166 of positive phase with the portion 164 being shown along theabscissae between a negative value of T_(i) and 0 indicating thatincreasing flow rates of carrier fluid causes a decrease in T, resultingin a negative phase for dT/df.

From an examination of the curves 154 and 156, it can be understood thatthere is an approximate reduction in the flow-rate-dependent temperatureerror of the heat of adsorption detector 10 in the embodiment of FIG. 3of four to one with a 90° phase angle when the inlet temperature isslightly positive as indicated at 162, compared to when Ti = 0.

Firstly, the reason for the reduction in the flow-rate-dependenttemperature error can be understood from an examination of equation (6)in the light of the curve 154 and 156. Increasing the temperature of thecarrier fluid entering the heat of adsorption detector 10 increasesT_(av) in a positive direction so that the term of equation (6) thatincludes T_(av) balances the other flow-rate-dependent term of theequation, resulting in a reduction in the flow-rate-dependenttemperature error. The inlet temperature at point 162 in the graph 150(FIG. 7) is the temperature that balances equation (6).

Secondly, the reason for the reduction in the flow-rate-dependenttemperature error can be understood from a consideration of the curve132 (FIG. 5) in the light of the curves 154 and 156 (FIG. 7). The heatof adsorption detector 10 normally operates in the region 138 of curve132 (FIG. 5) of rapidly decreasing spurious T with respect to flow ratef. When the temperature of T_(i) of incoming carrier fluid is increasedto the point 162 of graph 150, the region 136 of curve 132 is shifted tothe operating rate of flow of the heat of adsorption detector 10. Thisoccurs because increasing the flow rate f causes the higher heat levelfrom the warmer inlet to flow down to the second thermistor with lessreduction in temperature from the heat loss to the walls. This effect isshown by the curve 154 (FIG. 7) in which an inlet temperature equal toor below that of the body temperature T_(o) of the heat of adsorptiondetector results in a decreasing dT/df as shown by the portion 158 ofthe curve 154 and an inlet temperature above that of the bodytemperature T_(o) of the heat of adsorption detector results in anincreasing dT/df as shown by the portion 160 of the curve 154.

The graph 152 of FIG. 8 includes a curve 168 and a curve 170, with thecurve 168 indicating the magnitude of variation in the spurious dT/dfwith variations in the power added to thermistor 28 to increase the heatH₁ emitted by this thermistor and with the curve 170 indicating thephase of the spurious dT/df with respect to variations in the poweradded to thermistor 28 to increase the heat H₁ emitted by thisthermistor.

The curve 168 includes three portions, which are: (1) a portion 172 ofdeclining dT/df with increasing power added to the thermistor 28,occurring at low additions; (2) a portion 174 of increasing dT/df withincreasing power added to the thermistor 28, occurring at high poweradditions; and (3) a portion 176 of minimum dT/df occurring at amountsof power added to the thermistor 28 between the amounts added forportion 172 and portion 174. The curve 170 generally indicates anincreasing phase angle between the variations in flow rate and thespurious dT/df.

From an examinaton of the curve 168, it can be understood that there isan approximate reduction in the flow-rate-dependent temperature error ofthe heat of adsorption detector 10 in the embodiment of FIG. 2 of threeand one-quarter to one when sufficient power is added to thermistor 28to operate in the portion 176 of curve 168.

In FIG. 9, there is shown a block diagram of an interferometricrefractometer 180 having a polarized light source 182, a beam splitter184, a quarter wave plate 186, a beam combiner and phase comparator 188,a recorder 190, and a fluid-characterstic measuring station 192.

The light source 182, beam splitter 184 and quarter wave plate 186 arearranged with respect to each other so that light from the light source182 is split by the beam splitter 184 into two beams having the samephase, one of which passes through the quarter wave plate 186 beforebeing applied to the fluid-characteristic measuring station 192 and theother of which is applied directly to the fluid-characteristic measuringstation 192 through an equal optical-length path. The two beams aretransmitted through different flow streams within thefluid-characteristic measuring station 192. The beam combiner and phasecomparator is positioned with respect to the fluid-characteristicmeasuring station 192 to receive both beams of light after the two beamsof light have passed through the fluid streams and is electricallyconnected to the recorder 190 to record a signal which results from thecombining and phase comparison of the beams of light.

This refractometer is described more completely in "A LaserInterferometric Differential Refractometer For Detection ofChromatographic Effluent and Measurement of Volume Elasticity ofLiquids" by H. F. Hazebroek, Journal of Physics E: ScientificInstruments, 1972, Vol. 5, pps. 180-185. The beam combiner and phasecomparator 188 as described in that publication, develops a signalrepresenting the refractive index of one of the two fluids in themeasuring station 192 and records that signal.

The fluid-characteristic measuring station 192, in the preferredembodiment includes two flow cells 194 and 196, together with apparatusto cause liquid to flow through the flow cells while beams of light arepassed through them and certain temperature control apparatus.

To cause fluid to pass through the flow cell 194, thefluid-characteristic measuring station 192 includes a source of effluent198, connected by tubing 200 to the inlet port of the flow cell 194,with an exit port being connected by tubing 202 to a drain for the fluid(not shown). In the center of the flow cell, the fluid flows through apath aligned with the longitudinal axis of the flow cell having windowson each side of it so that the first beam of light which passes throughthe quarter wave plate 196 passes through a substantial length of thefluid before being transmitted from the fluid-characteristic measuringstation to the beam combiner and phase comparator.

To cause fluid to pass through the flow cell 196, thefluid-characteristic measuring station 192 includes a reference solventsource 204, connected to the inlet port of the flow cell 196 throughtubing 206, with an exit port of the flow cell 196 being connected bytubing 208 to a drain for the fluid in a manner similar to the flow cell194. The flow cell 196 is constructed in a manner similar to that of 194so that it receives the second beam of light from the beam splitter 184and transmits it through a length of the reference solvent beforetransmitting it to the beam combiner and phase comparator 188.

To control the temperatures within portions of the fluid-characteristicmeasuring station 192 in such a way as to reduce flow-rate relatederrors in one embodiment, the fluid-characteristic measuring stationincludes a first heat exchanger 210 and a second heat exchanger 212 withthe flow cells 194 and 196 being within the first heat exchanger and theinlet tubing 200 and 206 immediately adjacent to the inlet ports of theflow cells 194 and 196 being also within the second heat exchanger 212.These heat exchangers may be similar to the heat exchangers 106 and 112in the embodiment of FIG. 3 and are controlled in temperature withrespect to each other by a temperature control circuit similar to thecircuit shown in FIG. 3.

The first and second heat exchangers, as shown in FIG. 9 include atemperature control unit 214, a temperature sensing device 216 withinthe first exchanger 210, a heater 218 within the first heat exchanger210, a temperature sensing device 220 within the second heat exchanger212 and a heater 222 within the second heat exchanger 212. The heatexchangers 210 and 212 control the temperature of the fluid in the inlettubing to the flow cells 194 and 196 with respect to the temperature ofthe body of the flow cells 194 and 196 to reduce the flow-rate-relatedtemperature deviations of the fluid within the flow cells whilecharacteristics of the fluid are being measured in the manner explainedin connection with FIG. 3.

In another embodiment, the fluid-characteristic measuring station 192includes heater coils 224 and 228 and temperature control equipmentwhich may be used either in cooperation with the heat exchangers 210 and212 or without them to reduce flow-rate related errors by heating thewalls of the flow cells 194 and 196. The amount of heat generated byeach heating coil is independently adjustable so that the walls of eachflow cell can be adjusted in temperature with respect to the other flowcell.

To adjust the temperature of the wall of flow cell 194, the heating coil224 is wound around the flow cell and electrically connected to atemperature control unit 226 in a manner similar to the arrangement forheating the walls of heat of interaction detector shown in FIG. 1.Similarly, to adjust the temperature of the wall of the flow cell 196,heating coil 228 is wound around it and electrically connected to atemperature control unit 230. The temperature control units 226 and 230are adjustable and transmit different amounts of electrical currentthrough their respective heating coils, thus controlling the temperatureof the walls of the flow cells.

To control the temperature of the fluid flowing through the flow cell194 in a third embodiment, the fluid-characteristic measuring station192 includes temperature control units 232 and 236 and heaters 234 and238 with the temperature control unit 232 being connected to a heater234 within the flow path of fluid flowing through the flow cell 194 suchas within the tubing 200 adjacent to the inlet port of the flow cell 194and with the temperature control unit 236 being electrically connectedto a heater 238 within the tubing 206 adjacent to the inlet port of theflow cell 196. Of course, the heaters may be adjacent to the tubing toheat the tubing or may be outside the tubing but in contact with a heatconductor within or adjacent to the tubing so as to be in thermalcontact with the fluid.

While two heat exchangers 210 and 212 are shown in FIG. 9, with heatexchanger 210 enclosing the flow cells 194 and 196 and heat exchanger212 enclosing the inlet tubing 200 and 206, more or fewer heatexchangers may be employed in a refractometer. For example, instead ofone heat exchanger for both flow cells, a different heat exchanger maybe provided for each flow cell. Similarly, a different flow cell may beprovided for the inlet tubing to each flow cell. Moreover, somebeneficial results are obtained from controlling only the temperature ofeither the flow cells, with the inlet tubing being at ambienttemperature, or of the inlet tubing, with the flow cells being atambient temperature.

In some instruments more flow cells or fewer flow cells are requiredthan in the embodiments described herein or a cell in which a referencefluid is held stationary in its passageway may be substituted for theflow cell through which the reference fluid flows. In other instrumentsother types of housings are required than in the refractometer of FIG.9. Such instruments may accordingly require more or fewer heatexchangers to provide the required flexibility of control. Similarly, itmay only be necessary to control a single housing or inlet to thehousing with a single heat exchanger for some applications.

Before operating the refractometer, to determine the refractive index ofmaterials in a flow stream, two known identical fluids are pumpedthrough the flow cells while the refractometer is operated and thetemperature devices are adjusted as the flow rates are varied for thefluids until there is a minimum indication of the difference in therefractive index between the two materials with the fluctuations in theflow rate. Once the proper temperatures are set, the refractometer isoperated to determine the refractive index of materials and thusidentify the materials in one of the flow streams.

In one embodiment, the refractometer is adjusted for minimum error fromvariations in flow rate of the fluids using the heat exchangers 212 and210 and not using the heating coils 224 and 228 nor the transducers 234and 238 for temperature adjustments. To adjust the refractometer forminimum error before using it in this embodiment, solvents are firstapplied from the effluent source 198 and the reference solvent source204 through their respective conduits 200 and 206 and through the flowcells 194 and 196. Both solvents are identical in this case so that anerror-free comparison of the refractive index would indicate nodifferences.

While these identical fluids are being pumped through the flow cells 194and 196, the light source 182 transmits a polarized laser beam throughthe beam splitter 184 which splits the beam into two parallel beams. Afirst of the beams is transmitted through the quarter wave plate 186which creates a circular polarization in it and then transmits itthrough the fluid in the first flow cell 194 to the beam combiner andphase comparator 188. The second beam from the beam splitter 184 isapplied through an optical-length path equal to the optical-length ofthe first path directly through the fluid in the flow cell 196 and thento the beam combiner and phase comparator 188.

The beams are combined in the beam combiner to obtain fringes from phasedifferences caused by differences in the refractive indices of thematerials in the flow cells and these fringes are sensed and recorded bythe recorder 190 to indicate the phase difference between the two beamsof light caused by differences in the refractive indices of these twoflow cells.

While the refractive indices are being measured with identical fluidsflowing through both flow cells, the rates of flow of the fluids arevaried. While the rates of flow are varied, the heat exchangers 210 and212 are adjusted in temperature with respect to each other and withrespect to an absolute temperature by the temperature control unit 214.This adjustment causes a difference in the changes in temperaturebetween the identical fluid streams passing through the inlet conduitsfrom the effluent source 198 and reference solvent source 204 and thetemperature of the walls of the flow cells 196 and 194, with thetemperature of the fluids which are controlled by the heat exchanger 212generally being higher than the temperature of the flow cells controlledby the heat exchanger 210. This mode of operation is similar to theoperation described in connection with FIG. 3.

As the flow rates are varied and the temperatures of the heat exchangersare adjusted with respect to each other, the difference in refractiveindex caused by variations in the flow rates is observed on the recorder190. The temperature is adjusted until the differences in refractiveindex as indicated on the recorder 190 are at a minimum and then thesettings are maintained by the temperature control unit 214.

Once the proper temperatures between the heat exchangers 210 and 212have been located and set, the effluent, which is being investigatedreplaces the pure solvent from the effluent source 198 and therefractometer is operated. During its operation, the refractive index ofthe effluent is obtained by a comparison of the known refractive indexof the reference solvent which comparison is indicated by the recorder190 with a minimum of error due to fluctuations in the flow rate of thefluids.

It is not completely understood why fluctuations should occur in therefractive index due to variations in the flow rate but it is believedthat these are due to temperature effects. Changes in temperature causedby changes in flow rate affect the refractive index of many materialsand since changes in flow rates occur between the effluent source andreference source the comparison of refractive index between the twomaterials are effected.

It is believed that the proper differance of the temperature between theinlet fluid and the housing causes the flow-rate related temperatureeffects to reach a minimum when two such fluids are compared in a mannersimilar to that illustrated in FIG. 7 for the heat of absorptiondetector although the equations for heat loss from the heat ofabsorption detector would not be identical to those for therefractometer. Nevertheless, by analogy, the same type of curve isexpected by balancing the temperature errors caused by variations offlow rate which increase with an increase in flow rate against thosethat decrease with an increase in flow rate and vice-versa by adjustingthe heat flow paths within the flow cells.

The heating coils 224 and 228 reduce flow-rate-dependent errors inmeasurements of characteristics of fluids in a manner analogous to theheat exchangers 210 and 212 and analogous to the heating coil 15 in theembodiment of FIG. 1 of the heat of absorption detector. They may beused either separately from or in conjunction with the heat exchangers210 and 212 to provide individual adjustment of the temperature of thebulk of the flow cells 194 and 196, thus adjusting the temperature ofthe flow cells with respect to each other as well as the temperature ofthe flow cells with respect to the fluid entering the flow cell. Whenused in conjunction with the heat exchangers 210 and 212, they provide adifferential between the two flow cells while the heat exchangers 210and 212 aid in controlling the temperature difference between the fluidentering the flow cells and the flow cells.

To adjust the temperature of the flow cells, the amount of currentpassing through the heating coils 224 and 228 is adjusted by means ofthe temperature control units 226 and 230 respectively, thus adjustingthe heat generating in the coils and the temperature at which the flowcells are maintained by the heating coils. While temperature adjustmentsare made, the same identical fluids flow through each flow cell andchanges in the index of refraction indicated by the recorder 190 areobserved. The settings are experimentally determined in this mannerwhile the flow rates of the fluid fluctuate until differences betweenthe refractive indices of the two fluids with fluctuations in flow ratesare at a minimum. The refractometer is then operated to determine therefractive index of an unknown substance from the effluent source 198.

A third type of adjustment shown in FIG. 9 utilizes the transducers 234and 238 to control the temperature of fluid entering the flow cells.These transducers are heaters immersed in the fluid or formed as heatingelements immediately around the tube adjacent to the flow path to causeheat to be conducted into the flow path. The amount of heat given off bythe transducers 234 and 238 is adjusted by the temperature control units232 and 236 respectively, which may include potentiometers or the likefor adjusting the flow of current applied to the heating elements.

The transducers 234 and 238 are adjusted while the identical solventsare pumped through the flow cells with a fluctuating flow rate until therefractive index difference between the two fluids as indicated on therecorder 190 is a minimum. Once this adjustment is reached, an unknowneffluent may have its index of refraction determined by pumping itthrough the effluent source 198 and comparing it to the index ofrefraction from the solvent in the reference solvent source 204 asindicated by the recorder 190. This individually adjustable temperaturetechnique can be used in conjunction with the heat exchangers 210 and212 in the same manner as the individually adjustable heating coils 224and 228 may be used.

The individually adjustable heaters for individual flow cells shown inFIG. 9, which are heating coils 224 and 228 and transducers 234 and 238may be used to control the temperature of one flow cell with respect toanother or to control one portion of one flow cell. For example, if thereference solvent is held captive and stationary in flow cell 196, it isonly necessary to reduce errors in measuring flow cell 194 and this canbe accomplished by controlling one transducer to heat the inlet fluidand/or one heating coil to control the flow cell 194. Similarly, theflow-rate related errors may be reduced under some circumstances bycontrolling only the temperature of the housing or the temperature ofthe fluid.

While instruments have been described to identify unknown substancescarried by a fluid by their heat of adsorption or refractive index,other types of devices may be built in accordance with the invention orinventions which operate in other modes to utilize the invention of thisapplication or the invention of copending application Ser. No. 251,712referred to hereinabove. It is only necessary that: (1) a characteristicof a fluid or fluids is to be detected; (2) the detection of thecharacteristic is affected by temperature changes in the fluid; (3) thetemperature changes are created by flow rate changes of a fluid withrespect to its housing and/or with respect to another fluid in anotherflow stream; and (4) some of the components of the temperature changesincrease and others decrease with the same direction of change of therate of flow.

From the above description, it can be understood that the instrument ofthis invention has the advantages over prior art instruments of: (1)providing consistent and reproducible reductions in flow-rate-dependenterrors in measuring a characteristic of a fluid; (2) being lesssensitive to fluctuations in the ambient temperature; (3) being easilyadjusted for minimum flow-rate-dependent errors when requird bydifferent conditions of measurements; and (4) being adaptable to a widerange of instruments for measuring the characteristics of fluid ratherthan being only usable with heat of interaction detectors.

Although preferred embodiments of the invention have been described withsome particularity, many modificatons and variations in the preferredembodiments may be made without deviating from the invention.Accordingly, it is to be understood that, within the scope of theappended claims, the invention may be practiced otherwise than asspecifically described.

What is claimed is:
 1. A method of reducing flow-rate-dependent errorsin an instrument for measuring a characteristic of a fluid comprisingthe steps of:causing a fluid to flow through said instrument; varyingthe rate of flow of the fluid through said instrument; detecting therate of change of the flow-rate-dependent error in said characteristicmeasurement with respect to the rate of change of the flow of fluidthrough the instrument; and adjusting the flow of heat through theinstrument until the flow-rate-dependent error is at a relatively lowlevel; the step of adjusting the flow of heat through the instrumentincluding the step of adjusting the flow of heat until the factors thatcaused flow-rate-dependent increases are substantially balanced againstthe factors that caused flow-rate-dependent error decreases with thesame magnitude of change in the flow rate of the fluid by adjusting thetemperature of the fluid entering the instrument with respect to thetemperature of the body of the instrument while the rate of flow offluid is varied and the rate of change of the flow-rate-dependent erroris detected until the flow-rate-dependent error is relatively low.
 2. Amethod according to claim 1 in which the step of detecting the rate ofchange of the flow-rate-dependent error with respect to the rate ofchange of the flow of fluid through the instrument includes the stepsof:sensing and obtaining a first signal representing the magnitude ofsaid measured characteristic of the fluid at a first location during aplurality of different times; sensing and obtaining a second signalrepresenting the magnitude of said measured characteristic of areference substance at a second location during said plurality of times;subtracting the first signal from the second signal to obtain amagnitude difference indication for said characteristic for saidplurality of times; and determining the rate of change of the magnitudeof said characteristic with respect to the rate of change of said flowof said fluid from said magnitude difference indication and the rate ofchange of the flow of said fluid.
 3. A method according to claim 2 inwhich the step of adjusting the flow of heat includes the stepsof:controlling the temperature of the fluid entering the instrument withrespect to the temperature of the body of the instrument; and adjustingthe temperature of the fluid entering the instrument to a temperature atwhich the flow-rate-dependent error is relatively low and which isdifferent than the temperature of the body of the instrument.
 4. Amethod of measuring a characteristic of a solute in a solvent comprisingthe steps of:reducing the flow-rate-dependent error of an instrument bythe method of claim 3; causing the solvent containing said solute toflow through said instrument after the flow-rate-dependent error hasbeen reduced; detecting the characteristic of the solute as the solventflows through said instrument.
 5. A method according to claim 4 inwhich:the step of causing a fluid to flow through said instrumentincludes the steps of causing a fluid a characteristic of which is to bemeasured to flow in a first steam through a first passageway in saidinstrument and a reference substance to be located in a secondpassageway of said instrument; the step of varying the flow of the fluidthrough said instrument includes the step of varying the rate of flow ofone of said fluids through one of said passageways; the step ofdetecting the rate of change of the flow-rate-dependent error withrespect to the rate of change of the flow of fluid through theinstrument includes the step of comparing the same characteristic of thefluid in the first stream to the fluid in the second passageway whilethe said fluid in said first stream is varied in rate of flow withrespect to the fluid in said second passageway.
 6. A method according toclaim 5 in which the step of adjusting the temperature of the fluidentering the instrument includes the step of adjusting at least thetemperature of the fluid in one of said fluid streams entering theinstrument with respect to the temperature of the body of the passagewayof said instrument through which said stream flows while the rate offlow of at least one of said streams is varied and the rate of change ofthe flow-rate-dependent error is detected until the flow-rate-dependenterror is relatively low.
 7. A method according to claim 3 in which:thestep of causing fluid to flow through said instrument includes the stepsof causing a fluid a characteristic of which is to be measured to flowin a first stream through a first passageway in said instrument and areference substance to be located in a second passageway of saidinstrument; the step of varying the flow of the fluid through saidinstrument includes the step of varying the rate of flow of one of saidfluids through one of said passageways; the step of detecting the rateof change of the flow-rate-dependent error with respect to the rate ofchange of the flow of fluid through the instrument includes the step ofcomparing the same characteristic of the fluid in the first stream tothe fluid in the second stream while the said fluid in said first streamis varied in rate of flow with respect to the fluid in said secondpassageway.
 8. A method according to claim 3 in which the step ofadjusting the temperature of the fluid entering the instrument includesthe step of adjusting at least the temperature of the fluid in one ofsaid fluid streams entering the instrument with respect to thetemperature of the body of the passageway of said instrument throughwhich said stream flows while the rate of flow of at least one of saidstreams is varied and the rate of change of the flow-rate-dependenterror is detected until the flow-rate-dependent error is relatively low.9. A method according to claim 2 in which the step of reducing theflow-rate-dependent error of an instrument includes the step of reducingthe flow-rate-dependent error of a differential refractometer.
 10. Amethod according to claim 1 in which the step of detecting the rate ofchange of the flow-rate-dependent error with respect to the rate ofchange of the flow of fluid through the instrument includes the stepsof:continuously sensing and deriving a first signal representing themagnitude of the characteristic of the fluid at a first location;continuously sensing and deriving a second signal representing themagnitude of the characteristic of a reference substance at a secondlocation; continuously subtracting the first signal from the secondsignal to obtain a continuous magnitude difference indication; andcontinuously determining the rate of change of the characteristicdifference with respect to the rate of change of said flow of fluid fromsaid magnitude characteristic difference indication and the rate ofchange of the flow of said fluid.
 11. A method according to claim 10 inwhich the step of adjusting the heat includes the steps of:controllingthe temperature of the fluid entering the instrument with respect to thetemperature of the body of the instrument; and adjusting the temperatureof the fluid entering the instrument to a temperature at which theflow-rate-dependent error is relatively low and which is different thanthe temperature of the body of the instrument.
 12. A method of measuringa characteristic of a solute in a solvent comprising the stepsof:reducing the flow-rate-dependent error of the instrument by themethod of claim 11; causing the solvent containing said solute to flowthrough said instrument after the flow-rate-dependent error has beenreduced; and detecting the characteristic of the solute as the solventflows through said instrument.
 13. A method according to claim 1 inwhich:the step of causing a fluid to flow through said instrumentincludes the steps of causing a fluid a characteristic of which is to bemeasured to flow in a first stream through a first passageway in saidinstrument and a reference substance to be located in a secondpassageway of said instrument; the step of varying the flow of the fluidthrough said instrument includes the step of varying the rate of flow ofat least one of said fluids through one of said passageways; the step ofdetecting the rate of change of the flow-rate-dependent error withrespect to the rate of change of the flow of fluid through theinstrument includes the step of comparing the same characteristic of thefluid in the first stream to the fluid in the second stream while thesaid fluid in said first stream is varied in rate of flow with respectto the fluid in said second passageway.
 14. A method according to claim13 in which the step of adjusting the temperature of the fluid enteringthe instrument includes the step of adjusting at least the temperatureof the fluid in one of said fluid streams entering the instrument withrespect to the temperature of the body of the passageway of saidinstrument through which said stream flows while the rate of flow of atleast one of said streams is varied and the rate of change of theflow-rate-dependent error is detected until the flow-rate-dependenterror is relatively low.
 15. A method of reducing theflow-rate-dependent error of an instrument comprising the stepsof:causing the fluid to flow through said instrument; varying the rateof flow of the fluid through said instrument; detecting the rate ofchange of the flow-rate-dependent error with respect to the rate ofchange of the flow of fluid through the instrument; and adjusting theflow of heat through the instrument until the flow-rate-dependent erroris at a relatively low level; the step of adjusting the flow of heatthrough the instrument including the step of adjusting the flow of heatuntil the factors that caused flow-rate-dependent error increases aresubstantially balanced against the factors that causedflow-rate-dependent decreases with the same change in the flow rate ofthe fluid by adjusting the amount of power released by at least one heatemitting device in thermal contact with said fluid.
 16. A methodaccording to claim 15 in which the step of detecting the rate of changeof the flow-rate-dependent error with respect to the rate of change ofthe flow of fluid through the instrument, comprises the steps of:sensingand deriving a first signal representing the magnitude of acharacteristic of the fluid at a first location during a plurality ofdifferent times; sensing and deriving a second signal representing theamplitude of the characteristic of a reference fluid at a secondlocation during said plurality of times; subtracting the first signalfrom the second signal to obtain amplitude difference indications forsaid plurality of times; and determining the rate of change of saidamplitude difference with respect to the rate of change of said flow ofsaid fluid from said amplitude difference indications and the rate ofchange of said flow of said fluid.
 17. A method of measuring theamplitude of a characteristic of solutes in a solvent comprising thesteps:reducing the flow-rate-dependent error of an instrument by themethod of claim 16; causing the solvent containing said solutes to flowthrough said instrument after the flow-rate-dependent error has beenreduced; and detecting the amplitude of the characteristic of thesolutes as the solvent flows through said instrument.
 18. A methodaccording to claim 17 in which:the step of causing a fluid to flowthrough said instrument includes the steps of causing a portion of thefluid to flow in a first stream through a first passageway in saidinstrument and a portion of the fluid to flow in a second stream througha second passageway of said instrument; the step of varying the flow ofthe fluid through said instrument includes the step of varying the rateof flow of the first stream with respect to that of the second stream;the step of detecting the rate of change of the flow-rate-dependenterror with respect to the rate of change of the flow of fluid throughthe instrument includes the step of comparing the same characteristic ofthe fluid in the first stream to the fluid in the second stream whilethe said fluid in said first stream is varied in rate of flow withrespect to the fluid in said second stream.
 19. A method according toclaim 18 in which the step of adjusting the temperature of the fluidentering the instrument includes the step of adjusting at least thetemperature of the fluid in one of said fluid streams entering theinstrument with respect to the temperature of the body of the passagewayof said instrument through which said stream flows while the rate offlow of at least one of said streams is varied and the rate of change ofthe flow-rate-dependent error is detected until the flow-rate-dependenterror is relatively low.
 20. A method according to claim 15 in which:thestep of causing a fluid to flow through said instrument includes thesteps of causing fluid to flow in a first stream through a firstpassageway in said instrument and a reference substance to be located ina second passageway of said instrument; the step of varying the flow ofthe fluid through said instrument includes the step of varying the rateof flow of said steam through one of said passageways; the step ofdetecting the rate of change of the flow-rate-dependent error withrespect to the rate of change of the flow of fluid through theinstrument includes the step of comparing the same characteristic of thefluid in the first stream to the fluid in the second passageway whilethe said fluid in said first stream is varied in rate of flow withrespect to the fluid in said second passageway.
 21. A method accordingto claim 20 in which the step of adjusting the temperature of the fluidentering the instrument includes the step of adjusting at least thetemperature of the fluid in one of said fluid streams entering theinstrument with respect to the temperature of the body of the passagewayof said instrument through which said stream flows while the rate offlow of at least one of said streams is varied and the rate of change ofthe flow-rate-dependent error is detected until the flow-rate-dependenterror is relatively low.
 22. A method of measuring a characteristic of asolute in a solvent comprising the steps of:causing fluid to flowthrough a measuring instrument; adjusting the temperature of the wallsof the measuring instrument while varying the rate of flow of saidfluid; sensing and deriving a first signal representing thecharacteristic of the first fluid at a first location during a pluralityof different times as the rate of flow of the first fluid is varied;sensing and deriving a second signal representing the characteristic ofa second fluid at a second location during said plurality of differenttimes; comparing the first signal with the second signal to obtaincharacteristic difference indications for said plurality of times;determining the rate of change of the characteristic difference withrespect to the rate of change of said flow of said first fluid from saidcharacteristic indications and the rate of flow of the first fluid forsaid temperatures, whereby the temperature difference between the wallsand the first fluid providing the minimum change in the characteristicduring changes in the rate of flow of the first fluid is determined;causing the first fluid which includes the solvent and said solutes toflow through the instrument while the walls of the instrument and thefluid inlet temperature are maintained at said temperature producing theleast erroneous change in measured characteristic as the rate of flow ofthe first fluid is varied by heating said walls of said instrument andheating the inlet fluid to maintain the required temperature difference;and detecting the characteristic of the solute as the solvent flowsthrough said instrument whereby said instrument includes a minimumflow-rate-dependent error.
 23. A method according to claim 22 in whichthe step of detecting the characteristic of the solute includes the stepof detecting the refractive index of the first fluid in a differentialrefractometer.
 24. A method according to claim 23 further including thesteps of:varying the temperature of the fluid entering the instrumentwith respect to the temperature of the body of the instrument; andadjusting the temperature of the fluid entering the instrument to atemperature at which the flow-rate-dependent error is relatively low andwhich is different than the temperature of the body of the instrument.